Vacuum chambers are commonly used in the manufacture of semiconductor wafers. The vacuum chamber provides a controlled environment for the microelectronics circuit manufacturing in which such processes as chemical vapor deposition, aluminum sputtering, plasma etching, or plasma ashing occur. In order for these processes to take place in high yield and with little contamination, the residual gases in the vacuum environment must contain low concentrations of contaminant gases. A typical example of a deleterious gas or vapor in a vacuum environment is water vapor. Great care and effort are used to preclude water from the vacuum environment. Examples of techniques used to this end include baking vacuum chambers at high temperature to remove moisture adsorbed on vacuum chamber walls, vent purging vacuum chambers with inert gases prior to exposure of the chamber to an air environment, minimizing the time of air exposure of the chamber to atmospheric gases, and use of load-lock chambers to pre-treat wafers in a reduced pressure environment prior to their transfer to a high vacuum environment.
Load locks are vacuum chambers which remove the bulk of moisture and other adsorbed gases on substrates by reducing the pressure of the load lock chamber to pressures in the range of 10 to 0.001 torr. The amount of water vapor entering the load lock chamber can vary depending upon the number of wafers placed into the load lock chamber, the time to transfer the wafers into the load lock, the processing history of the wafers, and the concentration of water vapor in the ambient air. Removing the water vapor from the wafers and load lock before transferring the wafers to a central wafer handler or other vacuum chambers for further processes essential for high yields and uniformity of subsequent thin film forming and etching processes.
To prevent wafers with high levels of moisture from entering the central wafer handler, it has been previously proposed to measure the moisture content of the load lock chamber with a capacitive hygrometer sensor mounted to the load lock chamber. This is not desirable since the operation of this sensor relies upon diffusion of the contaminant gas through the chamber to the sensor for detection. While diffusion of gases is fast, the size of the chamber and spacing of substrates within the chamber prevent the gas in the chamber from being well mixed during the short time the chamber is opened. The concentration of a gas or vapor measured by the sensor will not be lndicative of the true concentration or load of gas in the chamber. A sensor located in the chamber may not achieve a zero state after a vacuum cycle since the vacuum level and time may not have been sufficient to remove all the gases or vapors from the chamber. Other sensors which could be suitably mounted to the chamber include a quartz microbalance sensor or a surface acoustic wave sensor.
Capacitive hygrometers and quartz microbalance sensors are commonly used as an equilibrium sensor in a flowing gas stream. In this use the sensor must be at a constant gas pressure. While the sensor output of the equilibrium gas concentration is not dependent upon gas flow, changes in flow will give changes in sensor output until the system is equilibrated.
Residual gas analyzers and other mass spectrometric based gas sensors could be attached to the chamber, however these devices are expensive and require highly trained and dedicated personnel for their use.
U.S. Pat. No. 6,125,687 proposes using a quartz microbalance sensor to detect outgassing of volatile materials placed in a vacuum environment, however, it requires the use of mechanical mixing elements in the chamber, a heat source to enhance outgassing of the material, and cooling elements in proximity to the sensor to enhance detection and condensation of the gas on the sensor. These enhancements are expensive and cumbersome for a semiconductor manufacturing environment. Additionally the presence of a mixing apparatus in the vacuum chamber creates particles which are intolerable for modern semiconductor manufacturing processes.
U.S. Pat. No. 5,170,057 describes an optical method for measuring the moisture content of gases in a reduced pressure environment. The method requires that electromagnetic radiation from a source transverse the chamber to the detector where the change in intensity due to absorption is related to the concentration of moisture in the chamber. Because wafers and wafer handlers are in the chamber, it is difficult to have the electromagnetic radiation transverse the entire chamber thus limiting the positioning of the sensor, the optical path length and ultimately the detection limit of the system. Additionally, ultraviolet light sources are required for moisture detection which requires additional cost of special safety shielding to prevent operator exposure to the radiation source.
In a typical semiconductor manufacturing process, wafers stored in the fabrication area are loaded into the load lock chamber of a wafer manufacturing tool. The load lock chamber is isolated from the vacuum pump by a valve located in a conduit called the fore-line. The fore-line connects the vacuum pump with the chamber and is generally a 1-2 inch diameter pipe for good conductance of gases to the vacuum pump.
In use the load lock is vented from a reduced pressure to atmosphere pressure with a dry inert gas like nitrogen. The load lock chamber is opened and wafers are loaded into the load lock chamber and the chamber is sealed. The valve in the conduit isolating the load lock chamber from the vacuum pump is opened allowing the gas in the load lock to be removed and the pressure in the chamber reduced. When the pressure in the load lock reaches a predetermined level in the range of 0.01 torr to 10 torr, the valve in the conduit between the vacuum pump load lock chamber is closed and wafers are moved from the load lock to the central wafer handler. Wafers are moved by the central wafer handler to different chambers for processing and then returned to the load lock. When all the wafers in the load lock have been processed and returned to it from the central wafer handler, the central wafer handler is isolated from the load lock, the load lock with the processed substrates is vent purged with dry inert gas to atmospheric pressure and the substrates removed from the tool.
During normal operation of a vacuum chamber, the pressure sensor of the vacuum chamber does not provide an indication of an increased water load as measured by a higher than normal base pressure. This is due to the high pumping capacity of modern vacuum pumps and the relatively low partial pressure of gases like water in the chamber. In this case it is possible for wafers with high moisture concentration to be transferred to the central wafer handler. Contamination sensors located in the chamber will only sense gases which are located in the vicinity of the sensor. During chamber evacuation, contamination sensors mounted to or inside the chamber experience a flow of gas away from the sensor and are not capable of measuring the true contaminant gas load of the chamber and substrates as the pressure is reduced. In an optimized vacuum system the contaminant gas load of the vacuum system should be measured in real-time during the pressure reduction step of the vacuum chamber in order to prevent contamination.
What is needed is a method and apparatus for measuring the contaminant load of a vacuum chamber and its contents in real-time. The process and apparatus of the present invention provides a measure of the contamination content of a vacuum chamber during the evacuation process.